In recent years, budget airlines in the U.S. have slowly drawn more and more travelers away from the Big Three (United, Delta, American), with many travelers opting for no-frills fares and à la carte pricing models instead. While the prices may be low, happiness with the experience seems to be, too: This year's American Customer Satisfaction Index surveyed more than 7,000 travelers, and found that low-cost carriers rank the lowest in airline customer satisfaction. More scary news? One low-cost company, in particular, may not be as safe as travelers think.

According to an investigation by the Tampa Bay Times, Las Vegas–based Allegiant's planes are four times as likely to fail during flight as those operated by other major U.S. airlines: In 2015, 42 of the company's 86 planes broke down at least once. The jets were forced to land 77 times for serious mechanical failures. The main causes? Failing engines (15), overheating tail compartments (nine), and smoke—or the smell of something burning (six).

There are several reasons to consider: The company's jets are old—averaging 22 years, a full decade older than the industry average of 12. Even more specifically, more than half of Allegiant's fleet are the MD-80 series, bought already used from foreign carriers. Only two other airlines—American and Delta—still use the MD-80 as part of their regular service, and Allegiant’s MD-80s break down twice as often. (Allegiant says it will phase out the MD-80s altogether by the end of 2019.)

Next up? The company's repair process. According to the Times report, after certain systems on Allegiant planes fail, the planes return to service, only to see...the same problems again. Eighteen times last year, "key parts"—think engines, sensors, and electronics—failed in-flight, were investigated, and then failed again, resulting in another unexpected landing. (Confronted with the statistics, Allegiant executives admitted the company could do better: “I can’t sit here and say that you’re wrong,” CEO Maurice Gallagher Jr. said. “We’re very much focused on running a better operation.")

But how can a fleet with so many issues continue to be cleared to fly? That answer lies in the Federal Aviation Administration's review policy, which examines each airline independent of the others, noting that the differences in the types of planes a company flies—and the way they fly them—make it useless to compare, say, Southwest to Delta, or American to JetBlue. The winds of change may be blowing, however: According to a 2013 report, 75 percent of inspectors polled by the U.S. Department of Transportation said comparing airlines directly would help make air travel safer.

Still, despite the criticism over its safety, Allegiant has found a niche market that seems to be working: Flying to 21 airports across the country, and with some seasonal routes, the carrier in 2015 posted a net profit of $220.4 million, more than double its 2014 net result of $84 million. According to AviationGuru.com, this makes Allegiant the most profitable airline of 2015.